Default template type vs default value

Question

This question follows the previous one .

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Case 1: Default Type

The following program does not compile or report error C2995: 'T foo(void)': function template has already been defined:

#include <iostream>
#include <type_traits>

template < typename T, typename = std::enable_if_t< std::is_integral<T>::value> >
T foo() { std::cout << "integral" << std::endl; return T(); }

template < typename T, typename = std::enable_if_t< !std::is_integral<T>::value> >
T foo() { std::cout << "non-integral" << std::endl; return T(); }

int main() {
    foo<int>();
    foo<float>();
}

Each of the patterns is alternately used and ignored (SFINAE), through two instances foo. Therefore, I assume that at some point the compiler sees:

template < typename T, typename = void >
T foo() { std::cout << "integral" << std::endl; return T(); }

template < typename T, typename = void >
T foo() { std::cout << "non-integral" << std::endl; return T(); }

Both definitions are the same, and the error is somewhat understandable. Perhaps it is less clear why the compiler did not assign different names of internal functions to this point.

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Case 2: default value

Now the program can be fixed, instead of using the default types, we use the default values:

template < typename T, std::enable_if_t< std::is_integral<T>::value>* = nullptr >
T foo() { std::cout << "integral" << std::endl; return T(); }

template < typename T, std::enable_if_t< !std::is_integral<T>::value>* = nullptr >
T foo() { std::cout << "non-integral" << std::endl; return T(); }

Here, following the same procedure, I get:

template < typename T, void* = nullptr >
T foo() { std::cout << "integral" << std::endl; return T(); }

template < typename T, void* = nullptr >
T foo() { std::cout << "non-integral" << std::endl; return T(); }

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int foo_int() { std::cout << "integral" << std::endl; return int(); }

float foo_float() { std::cout << "non-integral" << std::endl; return float(); }

int main() {
    foo_int();
    foo_float();
}

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Answer 1

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template < typename T, std::enable_if_t< std::is_integral<T>::value>*>
T foo();

template < typename T, std::enable_if_t< !std::is_integral<T>::value>*>
T foo();

void*. std::enable_if_t<std::is_integral<T>::value>* std::enable_if_t<!std::is_integral<T>::value>* . . T, .

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